Add disclaimer and release note v1.2.0 to README.md

Co-authored-by: rkttu <1297346+rkttu@users.noreply.github.com>

Author: Copilot

README.md

@@ -9,8 +9,15 @@ This package is a counterfeit of the .NET Framework version System.Messaging ass
 
 The source code for this package is excerpted from the .NET Framework reference source code.
 
+## Disclaimer
+
+This source code is based on Microsoft's Reference Source, but may contain differences from the original. Bug fixes have been applied in the public interest based on community-reported issues. If Microsoft officially releases an MSMQ client package for .NET Core/.NET 5+, that official package should take priority over this experimental package. Users are encouraged to migrate to the official package when it becomes available.
+
 ## Release Note
 
+### v1.2.0 (2025-11-30)
+Fixed Unicode string null termination in `StringToBytes` method. This bug affected `MessageQueue.Label`, `MessageQueue.MulticastAddress`, `Message.Label`, `Message.AuthenticationProviderName`, and queue format name properties, causing them not to persist correctly to MSMQ.
+
 ### v1.1.0 (2019-11-06)
 Trustee class namespace was moved from System.Messaging to Experimental.System.Messaging. It could break your existing build process.